The invention relates to window structures for the passage of X-rays and similar penetrating radiation and is in the nature of an improvement to the pending commonly assigned application U.S. Ser. No. 669,725 filed Mar. 23, 1976.
Window structures for penetrating radiation are known for X-ray tubes, where the high vacuum envelope contains a source of radiation which is to be transmitted by the window structure to the exterior of the envelope with as little loss as possible. Window structures are also known for permitting the entry of external radiation into the interior of a high vacuum envelope, that is so called inlet windows, and such window structures are used in connection with tubes for converting X-ray energy, or similar penetrating radiation of isotopes, such as gamma rays, into signals for the purpose of registration (for example for display and/or recording of a radiation image). Such tubes must be particularly vacuum-tight and free of gas molecules if conversion into electrons takes place within the tube, or if the converting elements, such as for example, photoelectric cathodes containing alkaline metals, are sensitive to atmospheric influences. Tubes of this type may contain, for example, measuring probes for producing electrical measuring signals, or they may contain arrangements which are suitable for converting the radiation into optical or electrical signals, from which a visible image may be obtained. Such tubes are known, for example, as picture-forming or image-converting tubes, television tubes, etc.
In the known tubes, metal windows which permit X-rays, gamma rays, etc., to pass through, frequently consist of beryllium (Be) which is quite permeable to the rays. The beryllium window plates are here soldered or welded to the housing of the tube by interposing connecting elements (compare U.S. Pat. No. 3,419,741 and British patent specification No. 978,878). However, in the present state of the art, beryllium window plates are not available in the desired range of sizes. Moreover, beryllium is capable of being shaped only with great difficulty. In many applications of X-ray windows, for example, for use in X-ray image intensifiers, a large diameter and freedom of shaping limitations are required in addition to the availability of a moderately priced material. Consequently the inlet windows in vacuum image intensifiers are at the present time still manufactured from glass, although this material absorbs X-rays to a substantial extent even when of the minimum thickness required for mechanical strength.
Thin foils consisting of titanium have also already been proposed as radiation-permeable windows (compare U.S. Pat. No. 3,878,417). The problem of securing thin foils is here solved by setting the metal foil in a stable frame which may be made of steel rings, for example. For the purpose of a vacuum-tight seal, the circumferential edge of the foil is welded to the adjoining surfaces of the rings. However, if a window structure for a vacuum tube is formed in such a way, the thin foil is forced inwardly into the interior of the tube by virtue of external atmospheric pressure, and consequently the tube must be of greater overall length than is required in a conventional image intensifier with an outwardly curved glass window.
Tubes consisting entirely of aluminum, that is of a light metal (of less than 4.5 grams per centimeter cubed) (4.5 g/cm.sup.3), have also not been proven successful. Up to the present time, no way has been found for maintaining a vacuum-tight seal about the necessary lead-in wires which would be acceptable in practical applications.
It is also known to weld shaped disks of light metal into a heavy metal frame so that windows even with a large diameter and of any desired shape can be obtained in an economical and simple manner (compare U.S. Ser. No. 607,874 filed Aug. 26, 1975). Plates of light metal other than beryllium are used here, and a layer of the light metal is fused with a layer of a heavy metal by rolling under high pressure. The heavy metal layer is then tightly welded to the frame. This is carried out, according to one embodiment, in such a manner that the edge of the window, which is bent outwardly to form an axial flange, is fitted into a ring-shaped frame, the edges of the window flange and frame being subsequently welded together.
In studying the present problem, it has been shown that it could be advantageous if, in securing the windows, it would be possible to dispense with the type of heating which is required in a conventional welding or soldering operation. For example, by not using any additional agents such as solder or adhesive, it would be possible to definitely prevent fluxing agents, or bonding and hardening agents, respectively, from later giving off gas molecules detrimental to the high vacuum condition which should be maintained within the tube.